Biol Philos (2011) 26:385–400 DOI 10.1007/s10539-011-9261-7 Why reciprocal altruism is not a kind of group selection Grant Ramsey • Robert Brandon Received: 10 October 2010 / Accepted: 7 March 2011 / Published online: 20 March 2011 Ó Springer Science+Business Media B.V. 2011 Abstract Reciprocal altruism was originally formulated in terms of individual selection and most theorists continue to view it in this way. However, this inter- pretation of reciprocal altruism has been challenged by Sober and Wilson (1998). They argue that reciprocal altruism (as well as all other forms of altruism) evolves by the process of group selection. In this paper, we argue that the original inter- pretation of reciprocal altruism is the correct one. We accomplish this by arguing that if fitness attaches to (at minimum) entire life cycles, then the kind of fitness exchanges needed to form the group-level in such situations is not available. Reciprocal altruism is thus a result of individual selection and when it evolves, it does so because it is individually advantageous. Keywords Reciprocal altruism Á Group selection Á Fitness Á Kin selection Á Game Theory Introduction Altruism has long been seen as the central problem of sociobiology (Wilson 1975a, b). The theoretical problem it raises is how does it evolve?, since altruism, by definition, lowers the fitness of the altruist. Empirically, the question is how prevalent is such behavior in nature? That question is not, of course, entirely separable from a proper theoretical understanding of the concept of altruism—we need to know exactly what it is that we are looking for in order to see how common it is in nature. The empirical issues will not be dealt with in this paper. G. Ramsey (&) Department of Philosophy, University of Notre Dame, Notre Dame, IN, USA e-mail: [email protected] R. Brandon Department of Philosophy, Duke University, Durham, NC, USA 123 386 G. Ramsey, R. Brandon Traditionally there have been three major responses to the theoretical conundrum raised by evolutionary altruism: group selection (Darwin 1871; Wilson 1975a, b, 1980; Wade 1978), kin selection (Hamilton 1963, 1964a, b; Maynard Smith 1964) and reciprocal altruism (Trivers 1971). Although these hypotheses are not mutually exclusive, they have been thought to be distinct, at least when originally presented. But for some time it has been argued that kin selection models of the evolution of altruism can be subsumed under group selection models (Michod 1982; Uyenoyama and Feldman 1980).1 This mathematical result should not be surprising. It is worth noting that Hamilton, the originator of kin selection theory did not like that term, which was introduced by Maynard Smith (1964), not Hamilton. Hamilton preferred the term ‘‘inclusive fitness’’ theory. Dispersing altruistic benefits non-randomly with respect to kin was simply one way, perhaps the likeliest way, of increasing inclusive fitness. To use Dawkins (1976) example, if the altruistic allele pleiotropically produces a green beard, then one can behave non-randomly with respect to beard color and the altruistic allele will evolve. But in either case the non-random behavior (or, more specifically, non-random dispersal of altruistic benefits) creates groups, often termed ‘‘kin groups’’, that compete with other groups. Furthermore, the explanandum of both theories is the same—a non-calculating form of altruism that requires no reciprocation for its evolution. It may apply equally well to plants2 (not known for cognitive sophistication) and humans. Reciprocal altruism, on the other hand, seems clearly distinct from group selection. Reciprocal altruism, as Trivers (1971) conceived it, evolves only within populations of animals that interact frequently and have mutual dependencies. So a moderately long life span and a population structure that brings the same individuals into contact with one another repeatedly is key. Trivers also argued that some cognitive capacity is favorable for the evolution of altruism.3 Under these circumstances, if I can help you at a small cost, c, and your benefit, b, is large relative to c, i.e., b c, then if you reciprocate with the same cost/benefit ratio, we both end up better off than we would have been had we not entered this coalition. Evolution will favor me if I have the propensity to engage in such coalitions and if I am good at picking out likely reciprocators. That is the basic idea and, it seems, it is individual phenotypes that are in competition. The phenomena explained by this theory—the sorts of altruism explained—are, it seems, quite different from that of group selection theory. For reciprocal altruism to 1 Whether that means they should best be interpreted as group selection is a matter with which we will not deal. It is also beyond the scope of this paper to deal with the related arguments for the unification of inclusive fitness and reciprocal altruism (e.g., Fletcher and Zwick 2006; Queller 1985). 2 Consider the following scenario. By sequestering nitrogen and diffusing it into the soil a plant may make the very local environment better for its type. But because of the nature of seed dispersal in this species, say big seeds dispersed by wind, its nearest neighbors tend to be closely related. Therefore the benefit dispersed, nitrogen, is dispersed non-randomly with respect to kin and this trait can evolve by ‘‘kin group’’ selection. As Hamilton stressed, it is a mistake to think that kin selection requires any sort of kin recognition. 3 Strictly speaking, the cognitive capacity is not a necessary condition for evolution of reciprocal altruism. Reciprocal altruism could evolve, for example, in extremely small populations where individuals have very few conspecifics to interact with (Axelrod and Hamilton 1981; Maynard Smith 1982), but such circumstances are far from the norm in nature. 123 Why reciprocal altruism is not a kind of group selection 387 be selected for it needs to be dispersed non-randomly with respect to the probability of reciprocation. (I’ll help you, but only if you are likely to help me.) Some have complained that Trivers’ theory takes the ‘‘altruism out of altruism’’, and that complaint has merit (e.g., Ghiselin 1974). In contrast, group selection theory explains a quite different phenomenon. As Wilson (1978) emphasized, there is a rift between reciprocal altruism and group-selected altruism—it is group selection but not reciprocal altruism that produces altruism with benefits that can be dispersed indiscriminately within the group and with no requirement of reciprocation. I’ll throw myself on a grenade to save my comrades in battle. However, the flip side of within-group altruism is, typically at least, between-group hostility. No one would expect the theory of reciprocal altruism to explain suicidal acts of altruism, while group selection might. Sober and Wilson (1998) disagree with this mainstream understanding of reciprocal altruism—they argue that it is just another form of group selection. So we want to note at the outset that their conclusion is, on the face of it, implausible given the two very different sorts of altruism involved. Their discussion of reciprocal altruism is subsumed within a discussion of evolutionary game theory. In game theory, they hold that ‘‘[f]or those who have become comfortable with the multilevel framework, it is child’s play to see the groups in evolutionary game theory, calculate relative fitnesses within and between groups, and determine what evolves on the basis of the balance between levels of selection.’’ (1998, 85) That is, iterated prisoners’ dilemma situations, the sort of situations Trivers originally had in mind in his model of reciprocal altruism, are really just another sort of group selection. Thus, they respond to the major theoretic problem of sociobiology by presenting an incredibly expansive theory of group selection—one that includes all of kin selection and all of what is covered by evolutionary game theory under group selection. They hold that altruism evolves only through a single mechanism, group selection. We think they go too far. Instead of maintaining that the only way that altruism can arise is group selection, we hold that altruism can arise via a plurality processes (reflecting the plurality of phenomena labeled ‘‘altruism’’). The goal of this paper is not to provide a complete account of these phenomena. Instead, our focus is rather narrow. We will argue that traditional reciprocal altruism, the sort of situation modeled by iterated prisoners’ dilemma, is not a form of group selection. Rather it is a form of individual selection.4 Our argument has a simple structure. Our main goal is to establish the following conditional: If fitness attaches to the whole life cycle,5 as opposed to some sub-part thereof, then reciprocal altruism is: (a) individually advantageous; and (b) evolves by individual level selection. We will be satisfied if the reader is convinced of the conditional’s truth. We do believe the antecedent of this conditional is true and will 4 Our argument against reciprocal altruism being group selection thus differs from other challenges to Sober and Wilson, like Godfrey-Smith’s (2008) argument that with the appropriate neighborhood structure (but without groups sensu Sober and Wilson), altruism can arise. Skyrms (1994) makes the related and more general point that all one really needs is correlated interaction. 5 More precisely, the smallest unit that fitness attaches to is the whole life cycle—fitness could attach to a unit greater than the individual life cycle and our argument would go through. 123 388 G. Ramsey, R. Brandon offer two arguments to that effect (‘‘The relationship between Altruism and fitness’’ and ‘‘Why reciprocal Altruism is not group selection’’). Thus our primary goal is modest—to establish the conditional. More ambitiously, we would also like to convince some that the antecedent is also true, and then invite them to perform modus ponens.